It would ease the problem of the fossil energy resource, such as the petroleum depletion by using renewable resources, such as cellulose, to produce liquid fuel and chemical products. The technology of using cellulose to produce liquid fuel and chemical products comprises steps of hydrolyzing cellulose to form a carbohydrate, such as an oligosaccharide or a monosaccharide, and fermenting the carbohydrate to produce liquid fuel and chemical products by using microorganisms. Typically, methods of hydrolyzing the cellulose to form sugars include dilute acid method, concentrated acid method and enzyme method; therein the concentrated acid hydrolysis method has advantages of low reaction temperature, high yields, and little sideproducts, and a disadvantage of high cost of acid recovery.
The process of concentrated acid hydrolysis of cellulose mainly comprised of two procedures: {circle around (1)} main hydrolysis process of hydrolyzing cellulose to oligosaccharides in concentrated acid, thereby obtaining main hydrolysate; and {circle around (2)} posthydrolysis process of hydrolyzing the oligosaccharides to glucose, thereby obtaining post-hydrolysate.
A common advanced process of concentrated sulfuric acid hydrolysis of cellulose is a low-temperature and two-stage sulfuric acid hydrolysis process. The process mainly includes firstly hydrolyzing hemicellulose of the plant cellulose materials to form monosaccharides which contain pentaglucose as a main component by using dilute sulfuric acid, secondly dissolving and hydrolyzing residual cellulose to oligosaccharides, for example cellotetrose and the like, by using 65-80 wt % sulfuric acid solution, and diluting and heating the resulting hydrolysate for a period of time to hydrolyze the oligosaccharide to glucose.
The concrete process of concentrated acid hydrolysis, such as the ARKANSAS University process described in U.S. Pat. No. 4,608,245 to Clausen et al. is described to include the following steps: mixing cellulosic material with 70-72 wt % sulfuric acid at a temperature of 50° C. for 10 minutes, and keeping the ratio of the sulfuric acid to the cellulose above 7.2; diluting the acid with water till the concentration thereof is between 40-50 wt %, and incubating at 90° C. for 20 minutes; separating lignin from the hydrolysate; performing a first extraction on the cooled hydrolysate by using a first extractant of C4˜C7 alkanols, such as heptanol, thereby obtaining an glucose-rich extraction raffinate and a first extract rich in sulfuric acid and the first extractant; then performing a second extraction to the first extract by using a second extractant of benzene, CCl4 or toluene, thereby obtaining an extraction raffinate merely containing water and sulfuric acid and an extract merely containing the first and the second extractants; then separating the first extractant from the second extractant by using a method of reduced pressure distillation to recycle the recovered acid, the first and the second extractants; finally, neutralizing remaining amount of sulfuric acid with lime, and filtering to obtain a liquid glucose without sulfuric acid. The most original of the process is using extraction method to separate sugars from acids. This principle seems to be very prefect. It's contemplated, however, that the technology has no practicability after a brief analysis. According to the preferable embodiment of the patent, the compositions of the hydrolysate obtained from the posthydrolysis process include 55% sulfuric acid, 40.5% water and 4.5% sugars, that is, the ratio of the sulfuric acid to the sugars is equal to 12.2. The compositions of the first extract include 79.4% heptanol, 14.5% sulfuric acid, 5.3% water and trace amount of sugars, that is, the ratio of the heptanol to the sulfuric acid is equal to 5.5. In the second extraction, the ratio of benzene to heptanol is about 5, that is, the quantity of benzene required to evaporate is 12.2×5.5×5=335.5 kg, and the quantity of heat wasted is 335.5 kg×434 kJ/kg=145607 kJ. However, complete oxidation of 1 kg glucose yields energy of only 15945 kJ, which is much smaller than the quantity of heat needed for evaporating the benzene. Obviously, it is a process without practicability, whose energy output is much smaller than the input.
It is obvious that, in the current concentrated sulfuric acid hydrolysis process, concentrated sulfuric acid solution with a concentration of 65-80 wt % is made one-off reaction with the cellulose materials. To ensure a substantial hydrolysis of the dissolved cellulose to monosaccharide in a complete hydrolysis process (namely the main hydrolysis process and the posthydrolysis process), the weight of cellulose hydrolysable per unit weight of sulfuric acid is very small (namely the weight ratio of the cellulose to the sulfuric acid is small), causing the costs of acid recovery too high for per kilogram of sugar obtained by hydrolysis. Furthermore, the water content of the main hydrolysate is higher. Moreover, it brings difficulty to recover the acid from the post-hydrolysate, thereby restricting the applications of the concentrated sulfuric acid hydrolysis that includes a lot of advantages.